Protective means for transformer tap changer



Sept. 14, 1965 o. P. M CARTY 3,206,559

PROTECTIVE MEANS FOR TRANSFORMER TAP CHANGER Filed Dec. 17, 1964 //v VENTOR. Ore/Nye CART), BY 0 ATTORNEY.

United States Patent Office 3,206,569 Patented Sept. 14, 1965 3,206,569 PROTECTIVE MEANS FOR TRANSFORMER TAP CHANGER Orin P. McCarty, 135 S. Mountain Road, Pittsfield, Mass. Filed Dec. 17, 1964, Ser. No. 419,065 1 Claim. (Cl. 200--61.03)

This application is a continuation-in-part of my earlier filed application Serial No. 219,901, filed Aug. 28, 1962, and assigned to the same assignee as the present invention.

This invention relates to protective apparatus for transformer load tap changing equipment and is in the nature of an improvement over that shown in Patent 2,246,182, granted June 17, 1941 on an application filed jointly in the name of the present inventor and Arthur Palme and assigned to the present assignee.

More particularly the invention relates to apparatus of this type using a vacuum switch as the arcing contactor component. Such a contactor contains the are completely, removing it from the usual liquid or gaseous insulation environments associated with transformers and thereby eliminates many of the design and maintenance problems resulting from arcing in these environments.

Arcing in air requires expensive arc chutes, or compressed air systems, and relatively long insulation paths which require protection from contamination by are byproducts, moisture, dust and the like. Arcing in oil produces hydrogen and hydrocarbon gases, carbon particles, tungsten carbide and copper particles. These by-products are detrimental to the insulating qualities of the oil and to the mechanical gears and bearings of the operating mechanism and require expensive materials and shields to give adequate insulation creepage and strike paths.

Arcing in askarel produces carbon, and hydrochloric acid, which is highly corrosive and detrimental to the insulating qualities of the askarel and solid insulation. Arcing in askarel can only be permtted when interrupting very small magnetizing currents and then only with use of inhibitors added to the askarel. Arcing in gasses such as sulfur hexafluoride generates undesirable quantities of white powdery by-products from the decomposition of the gas. This powdery material prohibits reasonable mechanical and insulation designs.

The elimination of these many problems by the use of a vacuum switch as the arcing contactor produces numerous highly desirable results such as virtually maintenance free load tap changing entirely within the main transformer tank or in an external compartment of much smaller dimensions, reduced insulation distance resulting in smaller size equipment, simpler mechanical systems requiring less motive power (a vacuum switch requires only an eighth of an inch gap compared with a two and one-half inch gap for arcing contactors under oil), greatly reduced costs because of smaller size and simplified construction and substantially reduced size and cost of the arcing contactor when mounted on the line end of a high voltage bushing such as in the system to which my Patent 2,360,147 is directed.

An object of the invention is to provide a new and improved apparatus for transformer tap changing under load.

Another object of the invention is to provide a simpler, more reliable apparatus for tap changing under load which is characterized by the use of a single arcing duty vacuum switch.

It is a more particular object of this invention to. provide a load tap changing apparatus for liquid insulated transformers which is of a design adapted to be immersed as one compact integral unit either wholly within the main transformer tank itself or wholly within a separate auxiliary tank of smaller dimension.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which FIG. 1 is a schematic circuit diagram of a transformer and tap changer embodying my invention, and FIGS. 2 and 3 are schematic illustrations of transformers showing the tap changing apparatus of FIG. 1 disposed, respectively, in the main tank and in an adjacent auxiliary compartment.

Referring now to FIG. 1 wherein is illustrated a presently preferred embodiment of the invention, there is shown therein a transformer 1, for use in power supply transmission or distribution circuits, having main windings 2 and 3, and a control supply winding 4. As all of these windings are closley inductively coupled, as is the practice with such transformer, they all have the same volts per turn. For regulating the voltage of the transformer 1, the winding 3 is provided with a plurality of taps 5 connected respectively to spaced tap contacts 6 which may conveniently be arranged on the arc of a circle. For changing connections between a main circuit conductor 7 and the taps 5 there is provided an integral tap changing apparatus including the tap contacts 6 and designated generally by the numeral 8.

As shown, the tap changing apparatus 8 comprises a pair of non-arcing duty selector contactors 9 and 10 for controlling the connections between the main circuit conductor 7 and a pair of branch circuit conductors 11 and 12 which are respectively connected through the halves 13 and 14 of a preventive reactor 15 to the individual contact fingers 16 and 17 of double finger non-arcing duty ratio adjuster switch, movable between tap contact-s 6. Inter-connecting the branch circuit conductors 11 and 12 is an arcing duty vacuum switch contactor 18 having a pair of normally closed contacts 19 in an evacuated envelope 20, one of the contacts 19 being movable relative to the other by means of an operating rod 21 passing through the wall of a flexible diaphragm or bellows 22 which is sealed to the envelope 20 for maintaining its vacuum. A spring 23 maintains the contacts 19 of the vacuum switch normally closed.

The entire tap changer 8 is physically located in a common ambient insulating medium such as an oil or askarel, either in the main transformer tank or in an auxiliary tank of a smaller dimension. The various switches and contactors are all mechanically interconnected to constitute a single integral mechanism and are operated in a predetermined sequence by a reversible motor such as capacitor motor 24, under the control of a voltage regulating relay 25, both of these elements being energized from across the control supply winding 4. FIG, 2 illustrates schematically the disposition of the entire tap changer 8 with the transformer 1 in a common tank 1a, both being immersed in a common body of insulating liquid 1b. FIG. 3 shows an arrangement wherein the tap changer 8 is located wholly within a separate compartment 8a adjacent to the main transformer tank, the compartment 8a containing a body of insulating liquid 8b in which the tap changer is immersed.

With the parts in their illustrated positions, the voltage to be regulated is normal as indicated by the position of the voltage regulating relay 25. If the voltage departs sufficiently in either direction from the normal value, the voltage regulating relay will complete one or the other of an operating circuit for the motor 24 causing it to rotate in the forward direction or the reverse direction, as the case may be. Mechanical operating connections of conventional type indicated by the dashed line 26 causes the motor to operate the contact fingers 16 and 17 in either direction from the position shown, and a shaft 27 from the motor 24 drives a scroll cam 28 for causing proper cyclic and sequential operation of the non-arcing duty contactors 9 and 10 and vacuum switch contactor 18.

The contactor 9 is operated by a push-pull rod 29, which includes an insulator 30, by means of a scroll cam follower 31 and the contact 10 is operated by means of a push-pull rod 32, which includes an insulator 33, by means of a scroll cam follower 34. The followers 31 and 34 engage the scroll cam 28 at diagrammatically opposite points thereon. The rods 29 and 32 are shown parallel to each other and are interconnected by a lever arm 35 pivotally connected at its ends to the rods 29 and 32.

The operating rod 21 for the vacuum switch 18 is connected through an insulator 36, to the midpoint of the lever 35, by means of a lost motion connection, shown for example as a pin 37 in the lever 35 and a slotted member 38 attached to the insulator 36.

The actuating cam 28 is illustrated by way of example as a rotatable cylindrical member having a peripheral camming slot within which are slidably positioned the cam followers 31 and 34. The camming groove is formed of a semi-circular circumferential land portion 28a (shown in dotted lines) extending for at least 180 degrees around the surface of the cam cylinder. Opposite ends of the land portion 28a are connected around the other side of the cam cylinder by a portion of the slot having a central circumferential dwell portion 28b axially offset from land portion and connected at its ends to opposite ends of the land portion by oppositely inclined rise portions 280 and 28d. In the normal closed position of the selector switches 9 and 10, the slotted cam 28 is positioned as shown in the drawing with the cam followers '31 and 34 located at opposite ends of the 180 degree land portion 28a of the cam slot.

It will be evident that in operation of the cam 28 a single 180 degree rotation from the position shown in either direction causes one or the other of the cam followers to move to the right as shown in the drawing thereby to open the associated selector switch 9 or 10. The cam follower which was thus moved into the dwell portion 28b of the cam slot remains momentarily in this position and then returns to its normal switch closing position in the land portion of the cam, all while the other cam follower remains in the land portion of the slot. During the next 180 degree rotation in the same direction, the other cam follower is similarly displaced and returned while the first follower remains stationary in the land portion of the cam. Thus in a complete single revolution of the cam 28, the cam followers 31 and 34 and their connected selector switches 9 and 10, respectively, are displaced to switch-opening position and returned in non'overlapping sequence.

The operation of the illustrated embodiment of the Invention is as follows:

With the parts in their illustrated positions, the circuit for the tapped winding 3 is from the main circuit conductor 7, through the contactors 9 and 10 in parallel through the branch circuit conductors 11 and 12 in parallel, through the two halves 13 and 14 of the reactor 15 in parallel, and through the contact fingers 16 and 17 in parallel to one of the tap contacts 6 on the winding 3. Due to the electrical symmetry of this circuit, the load current divides equally through the contactors 9 and 10, the windings 13 and 14 and the contact fingers 16 and 17. The windings 13 or 14 are interleaved or interlaced and are so connected that the magnetizing effects the load currents through them create cancel each other so that they interpose only negligible leakage reactance to the flow of load current. The vacuum switch 18 is virtually short circuited and carries no current.

If now voltage conditions change to such an extent that the relay calls for a tap change, the motor 24 will start to rotate in the proper direction. This will first cause either scroll cam follower 31 or scroll cam follower 34 to open contactor 9 or 10, depending upon the direction of rotation of the scroll cam 28. Opening of contactor 9 or 10 will cause no arcing at its contacts because the closed vacuum switch 18 provides a current conducting path between the branch circuit conductors 11 and 12, so that if contactor 9 opens, the current from the contact fingers 16, flowing in the branch circuit conductor 11, will flow through the vacuum switch 18, to the branch circuit conductor 12, so that all of the load current will then be carried through the contactor 10. Likewise if the contactor 10 opens, the current in the contact finger 17 and the branch circuit conductor 12, will be diverted through the vacuum switch 18, to the branch circuit conductor 11, so that all the load current will flow through the contactor 9.

The cam motion causing opening of the contactor 9 or the contactor 10 will also pivot the lever 35 around its connection to the operating lever 32 or 29, whichever remains stationary, thus moving the lost motion pin 37 to the right as viewed in the drawing. As this motion continues, the pin 37 reaches the end of the slot in the member 38 and vacuum switch contacts 19 are pulled apart, but such motion does not occur until after opening of the contacts 9 or 10 as the case may be. These contacts need be only separated physically by a relatively small distance in order to interrupt current and because they are completely sealed in the evacuated envelope 20 the interruption of current cannot contaminate the ambient insulating medium of the system with any deleterious products of the arcing. As soon as the vacuum switch 18 opens, current flow is interrupted in one or the other of the contact fingers 16 or 17 and this operation is so timed that after it occurs, the contact finger which carries no current moves out of engagement with a tap contact 6 and into engagement with the next adjacent tap contact 6. This is a so-called bridging or half-cycle position of the ratio adjuster switch which is followed immediately by the closing of the vacuum switch 18 due to the scroll cam 28 approaching the completion of a half revolution followed by the closing of whichever contactor 9 t r 10 had been opened so that after one-half revolution of the scroll cam 28 the cam followers 31 and 34 will be returned to the positions illustrated. Consequently the lever 35 and the contactors 9 and 10 and the vacuum switch 18 will be returned to the positions illustrated.

Under these conditions load current will divide between the contact fingers 16 and 17, but the flow of substantial circulating current produced by the tap to tap voltage difference between fingers 16 and 17 will be inhibited, because any such circulating current will flow through the halves of the reactor 15 in series, so that their magnetizing effects will be cumulative and the reactor 15 will interpose high reactance to the flow of circulating current.

In the half cycle or bridging position, the voltage of the main circuit conductor 7 will correspond to a voltage half-way between that of the two tap contacts 6 which are bridged by the contact fingers 16 and 17.

The tap change is completed by another half revolution of the scroll cam 28 in which the above sequence of operations is repeated, except that if contactor 9 had previously opened, then it will be contactor 10 which opens and vice versa. Such opening being followed by the opening of vacuum switch 18 due to the pivoting of the lever arm 35, and, as soon as the vacuum switch 18 interrupts current flow, the contact finger 15 or 17, as the case may be, will be moved to the-next adjacent contact 6, so that both contact fingers 16 and 17 will be engaging the next tap .contact 6, after Which the scroll cam re-closes the vacuum switch 18 and whichever contactor 9 or 10 is opened, so that after a complete revolution of the scroll cam 28 the parts will return to the positions shown in the drawing, except that the contact fingers 16 and 17 both will be engaging a next adjacent tap contact 6. It Will be seen that the lever 35 with its pin 37 and the slotted member 38 constitute a unitary double acting lost motion mechanical operating connection for the vacuum switch 18 from the operating means for the non-arcing duty contactors 9 or 10.

While a vacuum switch made with quality control should retain its vacuum and remain operative for many years, it is conceivable that it could fail in either closed position or its open position. If it should fail in its closed position throughout the tap changing cycle, then as the contactor 9 or opens and diverts current through the vacuum switch, the latter will not subsequently interrupt current through one or the other of the contact fingers of the ratio adjuster switch and such finger will then draw an arc and produce a gas bubble in the insulating liquid as it interrupts current in its branch circuit on moving to the next tap position. On the other hand if the vacuum switch should fail to open, the result will be that when contactor 9 or 10 opens it will draw an arc and produce a gas bubble as it interrupts the current flowing through its respective contact finger. In order to detect such failures, there is shown at FIGS. 2 and 3 a shed type barrier or hood 39 positioned above the load tap changing apparatus 8 to collect this gas. A standard gas detector relay 40 such as is used on most power transformers will then give an alarm after very few tap changes and before any appreciable pitting has occurred on the normally nonarcing duty contacts of the fingers 16 or 17 or contactors 9 or 10. It will be noted at FIGS. 2 and 3 that the hood 39 is located vertically above the entire unitary tap changing apparatus 8, and is closely adjacent the liquid surface, but that the hood does not extend appreciably beyond the apparatus 8 in any horizontal direction. The hood 39 therefore defines a space of small volume relative to the entire plenum space above the oil in its associated tank. For this reason generation of a small amount of gas may be promptly detected whereas the same quantity of gas would not be detectable if exhausted into the entire plenum space above the oil, especially in a large main tank.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention, and therefore it is intended by the appended claim to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

In a transformer tap changing apparatus, a tank containing an insulating liquid and providing a plenum space above the liquid; a transformer immersed in said liquid and having a winding provided with a plurality of taps; a main circuit conductor; a tap changer including an adjusting switch having a pair of stepping contact fingers immersed in said liquid, a vacuum interrupter switch connected between said contact fingers, and a pair of selector switches immersed in said liquid and each connected be tween one of said contact fingers and said main circuit conductor; said tap changer including a single unitary mechanism for actuaing said contact fingers and all said switches in predetermined sequence; a gas-collecting hood in said plenum chamber positioned close to the liquid surface and vertically above said contact fingers and selector switches, said hood defining a space significantly smaller than the full volume of said plenum space; and a gas responsive indicating device connected to receive only gas collected under said hood, whereby failure of said vacuum switch in closed position is detected by gas bubbles formed under said hood upon arcing at said contact fingers and failure of said vacuum switch in open position is detected by gas bubbles formed under said hood upon arcing at said selector switches.

References Cited by the Examiner UNITED STATES PATENTS 1,361,966 12/20 Colby 200-153 1,392,160 9/21 Hipple 200--153 1,642,405 9/27 Buchholz 200-61.03 2,112,064 3/28 Blume 32343.5 2,246,182 6/41 Palme et a1. 32343.5 2,360,147 10/44 McCarty 323-43.5 2,469,203 5/49 Palme et al 200-146 KATHLEEN H. CLAFFY, Primary Examiner.

ROBERT S. MACON, Examiner. 

